Various sealing and damping systems are used with rotating machinery, e.g., turbojet engines, steam turbines, gas turbines, compressors, and pumps, to prevent fluids under pressure from leaking along a shaft in the rotating equipment and to minimize vibratory motion of the shaft. Such systems frequently include labyrinth seals and squeeze film dampers.
Squeeze film dampers are frequently used with rotating equipment that utilize ball bearings. Squeeze film dampers use an oil film contained in a loose clearance space provided around the outer race of the ball bearing elements. The oil provides a hydrodynamic, viscous action that generates pressure in the oil film that opposes vibratory motion and dissipates vibratory energy of a rotating shaft. Squeeze film dampers have a disadvantage in that they may only be placed where the bearing elements are located on the shaft, and typically the vibration amplitude is relatively small at the location of the bearing elements. Another disadvantage of squeeze film dampers is their temperature limitation; squeeze film dampers are less effective at higher temperatures due to the temperature limitations of the working oil fluid.
Labyrinth seals are frequently used in rotating machinery to maintain a pressure differential between two adjacent portions on the exterior of a shaft. For example, labyrinth seals may be found in turbojet aircraft engines at each of the numerous turbine wheels along the longitudinal axis of the engine's shaft. There are numerous variations on labyrinth seals, but the typical labyrinth seal consists of a series of circular lands and annular grooves that present a tortuous flow path to the fluid. The lands and grooves of the typical labyrinth seal cooperate to provide minimum leakage while allowing radial or axial clearance between the shaft and associated stator or housing. While the labyrinth seal provides minimum leakage, the seal does not provide substantial damping of vibratory motion of the rotating shaft.
Another type of seal used to limit leakage of fluid between regions at different pressures along a shaft is the honeycomb seal. Honeycomb seals are frequently used in pumps and other types of rotating equipment employing incompressible fluids. Honeycomb seals typically utilize a stator with hexagonal cells lining the internal surface of the stator. The inner surface of the honeycomb-stator surrounds the shaft with a small clearance. The cells function to resist the flow of fluid past the cells. While there is some evidence that honeycomb-stator seals are more stabilizing than labyrinths seals in terms of vibration, honeycomb seals have the disadvantage of sometimes being damaged by high pressure drops.
Another problem experienced with rotating shafts and often caused by conventional seals is rotordynamic instability caused by cross-coupling due to fluid flow patterns around the shaft. In attempting to correct this problem, some manufacturers have added vanes upstream of a shaft seal that impose a fluid swirl opposite to the swirl induced by shaft rotation. This latter technique requires additional components which add weight to the overall system and can induce backward whirl of the rotor in certain speed ranges. Extra weight can be an undesirable, limiting factor in some situations, such as in turbojet engines. Thus, a need has arisen for a seal that prevents or greatly reduces fluid swirl about a rotating shaft while not requiring additional parts or components that add weight to the seal and are likely to cause more maintenance problems.
The performance goal for the next-generation aircraft engine is to double the thrust to weight ratio of present day engines. This will be achieved with increased operating temperatures and reduced weight. The increased operating temperatures will probably preclude the use of squeeze film dampers as damping devices. It is also desirable to reduce the vibration of the rotating shaft of jet engines for several reasons: (1) bearing life is increased by reducing the vibration; (2) reducing the vibration allows the blades to be placed with smaller clearances and thereby allows for greater efficiency; and (3) reducing the vibration reduces the noise of the engine because the noise associated with a jet engine is generally proportional to the amplitude of the vibrating shaft. The effectiveness of using squeeze film dampers is limited because the dampers may only be placed where bearing elements are located along the shaft. Labyrinth and honeycomb seals are not very effective for reducing the vibratory motion of the shaft.
Thus, a need has arisen for an apparatus and method for damping vibration of a rotating shaft under high temperatures and at locations that may be away from bearing elements. Furthermore, it would be advantageous to have a method and an apparatus that could seal as well as damp under these conditions. A similar need has arisen in other areas involving rotating machinery, such as steam turbines, gas turbines, compressors, and pumps.